In recent years, lithium ion secondary batteries have attracted attention in terms of reduction in size and weight for laptop computers, cellular phones, electric power tools, and electronic and communication devices, and recently, from the standpoint of application to environmentally-friendly vehicles, there has been strong demand for lithium ion secondary batteries for electric cars and hybrid cars, and in particular, for lithium ion secondary batteries with high voltage, high capacity, and high energy density.
A lithium ion secondary battery is composed of a positive electrode including a metal oxide such as a lithium cobalt oxide as an active material, a negative electrode including a carbon material such as graphite as an active material, and an electrolytic solution solvent mainly including carbonates, and is a secondary battery which is charged and discharged by the movement of lithium ions between the positive electrode and the negative electrode. More specifically, a lithium ion second battery is obtained by forming a positive electrode layer on the surface of a positive electrode current collector such as aluminum foil from slurry composed of the metal oxide and a binder with the positive electrode, and by forming a negative electrode layer on the surface of a negative electrode current collector such as copper foil from slurry composed of graphite and a binder with the negative electrode. Therefore, each binder has a role in adhesion between the active materials each other and between the active materials and the current collectors to prevent the active materials from peeling from the current collectors.
Conventionally, polyvinylidene fluoride (PVDF) with an organic solvent, N-methylolpyrrolidone (NMP) as a solvent has been used as a binder for adhesion of an active material to a current collector because of the high swelling resistance of the resin itself to the electrolytic solution, and used industrially for a number of models. However, this binder has the disadvantages of having poor adhesion between active materials each other and between the active materials and current collectors, requiring a large amount of binder for practical use, and as a result, decreasing the capacity and energy density of the lithium ion secondary battery. In addition, because of the use, for the binder, of NMP that is an expensive organic solvent, there have been problems with the price of a final product and with working environment conservation in the preparation of slurry or a current collector.
As a method for solving these problems, water-dispersible styrene-butadiene rubbers (SBR) combined with carboxymethyl cellulose (CMC) as a thickener have been proposed (for example, see Patent Literatures 1 to 3). This type of water-dispersible SBR has been used as a aqueous binder for an lithium ion secondary battery electrode in a wide range of application, because the SBR type dispersing element is inexpensive because of its water dispersibility, and advantageous from the standpoint of working environment conservation, and has favorable adhesion between active materials each other and between the active materials and current collectors. However, this binder has the problems of low mechanical stability, and of low swelling resistance to an electrolytic solvent. Furthermore, this binder has the problem of decreasing charge-discharge cycle characteristics in the case of a lithium ion secondary battery for use at high temperatures.
In order to prevent this decrease in charge-discharge cycle characteristics at high temperatures, Patent Literature 4 proposes a binder composed of a monomer derived from an ethylenically unsaturated carboxylic acid ester and an ethylenically unsaturated carboxylic acid. However, this binder is inferior in adhesion between active materials, and has the problem of significantly decreasing the adhesion between the active materials and current collectors. In addition, Patent Literature 5 proposes a binder obtained by mixing a binder composed of a monomer derived from an α,β-unsaturated nitrile compound and a (meth)acrylic acid ester with a binder composed of a monomer derived from a conjugated diene. However, this binder composition has a problem with adhesion between active materials while the binder composition has high charge-discharge high-temperature cycle characteristics.
Among water-dispersible type binders obtained by the previously studied methods as described above, any binder for a lithium ion secondary battery electrode has not been found which has favorable adhesion between active materials each other and between the active materials and current collectors, along with charge-discharge high-temperature cycle characteristics, and currently, PVDF and SBR with CMC as a thickener have been mainly used respectively in spite of having environmental problems and in spite of having problems in terms of charge-discharge high-temperature cycle characteristics.